Update lua-introduction.rst

readthedocs/lua-introduction.rst

@@ -368,26 +368,28 @@ Here is how a protected call is done::
     end
   end
   
-  -- call foo() in protected mode
-  status,returnvalue = pcall(foo, 'hello') -- this is like calling foo('hello')
+  -- call foo('hello') in protected mode
+  status,returnvalue = pcall(foo, 'hello') 
+  
   -- since this call should succeed, status will be true
   -- returnvalue should contain 4.2
   assert(returnvalue == 4.2)
   
-  -- call foo() again in protected mode
-  status, returnvalue = pcall(foo) -- call foo() without arguments
+  -- call foo() without arguments in protected mode
+  status, returnvalue = pcall(foo) 
   -- above will fail and status will be false
   -- But returnvalue will now have the error message
   
   assert(not status)
   print(returnvalue)
+  -- above prints 'message expected'
 
-The error handling mechanism works but has following issues:
+The Lua error handling mechanism has following issues:
 
-* The code that can raise errors must be a function as pcall() can only call functions
+* The code that can raise errors must be encapsulated in a function as pcall() can only call functions
 * The return values from pcall() depend upon whether the call terminated normally or due to an error - so caller needs to check the status of the call and only then proceed
 * On raising an error the ``longjmp`` unwinds the stack - there is no mechanism for any intermediate objects to perform cleanup as is possible in C++ using destructors, or in Java, C++, Python using ``finally`` blocks, or as done by the ``defer`` statement in Go
-* You can setup a finalizer on Lua user types that will eventually execute when the value is garbage collected - this is typically used to free up memory used by the value - but finalizers by their nature are not guaranteed to run in a deterministic manner
+* You can setup a finalizer on Lua user types that will eventually execute when the value is garbage collected - this is typically used to free up memory used by the value - but you have no control over when the finalizer will run, hence relying upon finalizers for cleanup is problematic